1. Field of the Invention
This invention relates to improved large drums for containing high-dispersion, high air content solids, and a method for filling them.
Handling pourable, high dispersion, and high air content solids of extremely low bulk density, such as, for example, high dispersion silicic acid (HDK), presents problems in various respects. Both manufacturers and ultimate consumers are faced with the problem that these substances cause dust with even the least amount of air convection. This dust generation should be avoided so that personnel, handling the product, are protected from any harm to their health which may be caused by breathing in the dust. Furthermore, the low bulk density results in increased transport costs, because the ratio of drum weight to filling weight is high, and a correspondingly large amount of packaging material is required.
Due to its spatial, three-dimensional branch structure, HDK is a product which has an extremely low tamped density of about 40 to 50 g/l. Moreover, the distinction must be made between the operating bulk density and the tamping density value according to DIN ISO 787/11. Due to its fine structure, HDK is capable of binding with a large amount of gas, for example, air, with the result that the product is put into a quasi-fluid state at about 20 to 30 g/l. This removable air fraction voluntarily escapes only very slowly and incompletely. This fluid state also increases the dust problem, since the mobility of HDK agglomerates is extremely high. This means, in production, that every conveying operation puts the HDK into this fluid state which then makes it more difficult to fill the drums, since the specific amount of time spent on filling each drum is increased, thus, in turn, reducing the capacity.
Pourable, high dispersion and high air content solids of extremely low bulk density, such as HDK, are therefore introduced into air-permeable bags predominantly with the aid of an externally applied vacuum. In this case, the filling duration rises with an increasing air content. The bags in this case consist of 3 to 4 layers of paper and, in addition, one layer of the paper may be lined with polyethylene (PE) as a barrier against penetrating moisture. In order to achieve the desired air permeability during filling, all the layers are microperforated. The advantage of this is that when the product is introduced into the bag, it is compressed and its filling density rises, as compared with the natural bulk density. It is also possible to carry out predeaeration by means of special press rollers, but this always results in structural damage to the HDK, which reduces its thickening property, or the thixotropic property, of the pyrogenic silicic acid.
Due to the higher weight of the product in the drum, transportation costs are reduced. However, this cost saving is at the expense of an extra outlay for procuring the special bags and the filling plant required for the bags.
2. The Prior Art
The described vacuum filling into multilayer, partially PE-lined paper bags is presently the general packaging standard for so-called "fumed silica". In this method, the problems of air permeability, dust tightness, drum stability, and moisture barrier properties are solved satisfactorily. These paper bags, by their very nature, are unsuitable for large filling quantities. Current filling quantities for drums of this kind are usually 10-20 kg.
In European Patent No. 0 773 159 or U.S. Pat. No. 5,682,929, there is disclosed a method and a container for the repeated filling and emptying with pourable product of low bulk density. The fabric container describes the so-called big bag or super bag that has a filling capacity of 90 to 350 kg. The fabric container consists of flexible air-permeable fabric, preferably a single-layer or multilayer synthetic fabric, with at least one accessible orifice. These fabric containers are likewise filled by means of vacuum-filling systems. Here, the fabric container is evacuated, and the product is drawn through the opened orifice into the fabric container until a predetermined filling weight is reached. During this time, the gas escapes, being distributed over the entire surface of the fabric container. During filling, the product is reversibly compacted, which is similar to filling the bags without its structure being destroyed.
These drums, however, have many disadvantages. For large flexible drums, paper bags cannot be used, since, on the one hand, they do not fulfill the necessary strength and transport safety requirements and, on the other hand, multiple use is not possible. Furthermore, there are no manufacturing plants and filling devices available for these paper bag sizes.
If the commercially available large flexible drums consisting, for example, of polypropylene fabric are employed, they can be used only for transporting filling substances insensitive to moisture, since the moisture barrier property of the fabrics used is insufficient to prevent an unacceptable increase in the moisture content of the HDK. The moisture barrier property is one of the main preconditions for the widespread use of these large flexible drums for all HDK types and areas of HDK use.